Step-by-step mechanisms are usually constructed as clamping roller switching mechanisms and have continuously variable operation to enable adjustment, for example, of a motor vehicle seat. A drive element and a driven element together with facing clamping surfaces define wedge-shaped clamping gaps that taper in opposite directions and in which clamping rollers are arranged. If the drive element is moved out of its starting position or its home position, the drive element performs a work stroke. The return movement of the drive element into the starting position is an idle stroke in which the driven element is not actuated. In the work stroke of the drive element in one direction, the first clamping rollers are brought into clamping engagement with the clamping surfaces, wherein the other clamping rollers are supported fixed to the frame and remain out of clamping engagement with the clamping surfaces. During the idle stroke, the previously clamped clamping rollers are out of clamping engagement with the clamping surfaces due to the reverse movement, and the other clamping rollers also remain supported on the stop fixed on the frame and out of clamping engagement with the clamping surfaces during the idle stroke. Thus, the driven element is not carried along during the idle stroke.
If the step-by-step mechanism is not actuated, it is ensured that a torque introduced from the outside into the driven shaft of the step-by-step mechanism is introduced into a housing, preferably a housing fixed to the frame. In the case of the motor vehicle seat, the weight of the seat or spring forces and/or the weight of a person generate the torque acting on the driven shaft from the outside. The step-by-step mechanism thus prevents the seat height from being changed unintentionally.
DE 10 2006 026 392 A1 discloses a step-by-step mechanism that is formed as a clamping roller switching mechanism and has an outer ring that can be connected to a drive element and an inner ring acting as the driven element. Between the outer ring and the inner ring there are clamping bodies that interact with double clamp ramps on at least one of the rings. Concentric to the rings there are mutually adjustable spring carriers. The spring carriers are clamped relative to each other by means of a spring in order to spring-load the clamping bodies.